The present invention relates to axle assemblies for driving a pair of independently rotatable axle shafts, and more particularly, to such an arrangement of the type in which differential gear means are provided to permit rotation of the axle shafts at different rotational speeds.
The present invention will be illustrated and described in connection with an axle assembly which is intended to be the rear-rear axle of a tandem axle arrangement. A "tandem" axle arrangement is one having two separate axle assemblies, for example, toward the rear end of the vehicle, wherein one axle assembly follows the other, in the direction of movement of the vehicle. However, those skilled in the art will understand that the present invention is not limited to use in an axle assembly which is part of a tandem axle arrangement, but may be used in any axle assembly, including independently rotatable, generally co-axial axle shafts, each shaft being driven by a separate ring gear.
U.S. Pat. No. 4,432,431, assigned to the assignee of the present invention and incorporated herein by reference, illustrates a typical front-rear axle assembly for use as part of a tandem axle arrangement. For purposes of understanding the present invention, and the background thereof, the device illustrated in the above-incorporated patent is relevant because it illustrates the conventional arrangement of such axles. In the conventional axles, a relatively high-speed, low-torque input is transmitted to a drive pinion which is in engagement with a ring gear. The ring gear transmits a relatively low-speed, high-torque motion to the input of an inter-wheel differential, which includes a pair of output side gears, each of which transmits one-half of the total output torque to each of the axle shafts.
Although such axle assemblies have been in widespread commercial use, and have been generally satisfactory in performance, such axles, having the overall configuration described above, have certain disadvantages. In the configuration described, the total drive torque, to be delivered to the wheels, is transmitted through a single ring gear, thus requiring that ring gear to be quite large and expensive. As a related point, the bearing sets which support that particular ring gear within the axle housing are also required to be quite large and expensive. Finally, the inter-wheel differential is subjected to the full driving torque (being disposed downstream of the drive pinion-ring gear reduction), thus requiring a larger, much more expensive differential gear set.
Accordingly, it is an object of the present invention to provide an axle assembly in which it is possible to use a smaller, less expensive, inter-wheel differential gear set.
It is a related object of the present invention to provide such an axle assembly in which it is possible to use ring gears and bearing sets which are smaller and less expensive than in the conventional axle assembly described above.
It is a further related object of the present invention to provide an axle assembly having a configuration whereby it is possible to reduce the overall axle package size, thus making it possible, where desired, to increase the ground clearance of the axle assembly.
It is another object of the present invention to provide an axle assembly which is capable of being substantially quieter than the conventional, prior art device by not transmitting the total drive torque through a single gear mesh.
U.S. Pat. No. 3,105,395, assigned to the assignee of the present invention, and incorporated herein by reference, illustrates a device known in the art as a "twin countershaft" transmission. The above-incorporated patent illustrates the basic concept of "torque-sharing", i.e., transmitting a particular drive torque through a device by means of two parallel torque paths, each transmitting approximately one-half of the total torque, rather than transmitting all of the torque through a single path, as was conventional. Among the related advantages of a "twin countershaft" type of device is the ability to balance various torques and reaction forces which exist in the device.
Although the advantages of the "torque sharing" principle used in twin countershaft transmissions have long been recognized, those skilled in the axle art have been previously unable to provide a practical, commercially satisfactory axle assembly utilizing the torque-sharing principle used in twin countershaft transmissions.
Accordingly, it is an object of the present invention to provide an axle assembly which utilizes the torque-sharing principles of twin countershaft devices, and which has the performance benefits inherent in twin-countershaft devices, as described above.
The above and other objects of the present invention are accomplished by the provision of an axle assembly of the type including independently rotatable, generally co-axial first and second axle shafts. First and second ring gear means are operable to drive the first and second axle shafts, respectively. First and second drive pinion gears are in meshing engagement with the first and second ring gear means, respectively. Included is a differential gear means having at least one input pinion gear in driven relationship with an input means, and first and second output gears in meshing engagement with the input pinion gear. The first output gear is operable to transmit torque to the first drive pinion gear, and the second output gear is operable to transmit torque to the second drive pinion gear.
In accordance with a more detailed aspect of the present invention, the first output gear transmits torque to the first drive pinion gear through a first parallel axis gear set, and the second output gear transmits torque to the second drive pinion gear through a second parallel axis gear set, the first and second parallel axis gear sets being substantially identical, and being arranged generally in a mirror image relative to an axis of rotation defined by the first and second axle shafts.
In accordance with a still further aspect of the present invention, the first output gear, the first drive pinion gear, the first ring gear means, and the first axle shaft define a first torque path. Similarly, the second output gear, the second drive pinion gear, the second ring gear means, and the second axle shaft define a second torque path. The first and second torque paths are arranged generally in a mirror image relative to an axis of rotation defined by the axle shafts.